The object which can claim to hold this title is probably one of the comets which passed close to the Sun many years ago
and has returned to the furthest limits of the Solar System never to return again.
If we limit our choice to objects that can be observed at a great distance,
then we can restrict our selection to objects that are 'permanent' members of the Solar family.
We must then consider the planets, periodic comets, and asteroids.

The Planets:

Pluto is the planet whose orbit takes it further from the Sun than any of the other planets.
Pluto's orbit is a much more elongated ellipse than are those of the other planets, and near its closest approach to the Sun,
Pluto is closer to the Sun than Neptune.
So for most of the time it is true that Pluto is the most distant planet, but until 1995 it was closer to the Sun than Neptune.

So, at the moment, Pluto is the furthest of the planets from the Sun.
Its distance is just over 39.5 AU. (An AU, or astronomical unit, is the mean distance of the Earth from the Sun,
which is approximately 150 million kilometres).

The possibility of there being another planet, the so-called Planet X, has been suggested.
The main scientific reason for another planet was that there were unexplained perturbations to the orbits of Uranus and Neptune.
A recent analysis of the motions of these two planets has shown
that the supposed perturbations are not real and that there is no need for another planet
and indeed there is no evidence for the existence of one.

Periodic comets:

Many periodic comets are known with orbits that lie inside the planetary system.
Halley's Comet is one of these.
There are only two comets which have been observed at more than one close approach to the Sun
that move further away from the Sun than Halley's Comet. These are P/Comet Swift-Tuttle and P/Comet Herschel-Rigollet.

Halley's Comet has been seen at many returns; there are written accounts of all of its returns during the last two millennia.
Halley has a period of 76 years and a maximum distance from the Sun of 35.3 AU.

P/Comet Swift-Tuttle has recently (September 1992) been rediscovered.
It has been identified with previous comets seen in 1737 and 1862.
It has a period of 135 years and a maximum distance from the Sun of 51.7 AU.

P/Comet Herschel-Rigollet has been seen twice in 1784 and in 1939.
It has a period of 155 years and a maximum distance from the Sun of 56.9 AU.

There are many comets that have only been seen to make one approach to the Sun.
Many of these have orbits that are very elongated.
With an orbit of this shape it is often very difficult to distinguish between an orbit which is closed
(meaning that the comet will eventually return towards the Sun) and one which is open (will never return).
These have orbits that take them out to more than 1,000 AU, but these large distances are very inaccurate.

Comets are believed to arise from a cloud of many millions of objects in circular orbits around the Sun
at distances of about 50,000 AU.
These objects would then represent the outermost reaches of the Solar System.

Comets although they are known to travel to these outer reaches of the Solar System,
can only be observed when they are close enough to the Sun to reflect enough light to allow them to be seen.
At large distances from the Sun,
the Sun's radiation is not strong enough to evaporate the ices that form the comet's solid core and so the comet has no coma or tail.
The solid cores are only about 10 kilometres across and so appear very faint at large distances.
Until 1994, no comet had ever been seen further than about the distance to Saturn,
but in January 1996 Halley's Comet was observed at a distance of about 15 AU.

Distant small bodies in the Solar System:

Most of the asteroids lie in a belt between the orbits of Mars and Jupiter.
It has been suspected that some of the matter from which the Sun and planets were formed
exists in a ring of small bodies much further away from the Sun than the planets.

A minor body, named 1992 QB1, was discovered in September 1992.
It was further from the Sun than any planet in the Solar System.
Its discoverers were discussing the novels of John Le Carré at the time and suggested the name Smiley.
Unfortunately there is already a minor planet with that name and so 1992 QB1 will remain the name of this distant object.
It is a very faint star-like object, visible in only the largest telescopes.
It is assumed to be an almost black object with a diameter of less than 200 kilometres.
Little is known of its orbit but it is about 37-59 AU from the Sun.
It is thought to be one of these 'left-overs' from the formation of the Solar System.
If so, it is likely to be one of thousands, or perhaps millions, of such objects.
The problem of finding objects like these is that they are extremely faint.
1992 QB1 is 6 million times fainter than the faintest object visible to the naked eye.

In March 1993 a second object, named 1993 FW (or unofficially Karla, after George Smiley's antagonist), has been reported.
It is very likely that this object is at a similar distance.
Since then, several more such objects have been found (two of them with the Isaac Newton Telescope).
These have distances from the Sun of between 34 and 45 AU.
The Hubble Space Telescope has been used to look for comet sized objects at these sorts of distances,
and the observers have reported that many such objects are present,
confirming that there is a belt of small objects associated with the bigger ones found earlier.
This is almost certainly the Kuiper belt that was predicted to exist from theoretical analysis of periodic comets with short periods.
These objects are thus more likely to be cometary objects than asteroids.

Produced by the Information Services Department of the Royal Greenwich Observatory.